Callisto is the second largest moon of Jupiter. It
has no atmosphere and is about the same size as the planet Mercury. It is composed of
approximately equal proportions of ice and rock. The moon's crust is heavily cratered with
remnant rings of enormous impact craters. The largest craters have been erased by the flow
of the icy crust over geologic time. Craters and the associated concentric rings are about
the only features to be found on Callisto.
The moon is probably composed of a large rocky
core surrounded by water and ice giving it a dark color. In places meteorites have
punctured holes in the crust causing water to spread over the surface forming bright rays
and rings around the crater.
